Method of producing iron chromium alloys of appreciable nitrogen content



Patented Feb. 2, 1937 UNITED STATES PATENT VOFFIC 2,069,205 METHOD orrnonncmc. moN cimoMmM ALLOYS V 'CONTENT OF APPRECIABLE NITROGEN WilliamB. Arness, Baltimore,-Md., assignor, by mesne assignments, to ltustlessIron and Steel Corporation, Baltimore, Md.,'a corporation of Delaware NoDrawing. Original applicationDecember 3,

1932, Serial No. 645,637.

Divided and this application January 4, 1934, Serial No. 705,282 7Claims. (01. 75-127) larly to corrosion-resisting or rustless alloyirons and steels of the class indicated, and to an art of producing thesame.

Among the objects of my invention are the production ina simple, directand economical manner of rustless' ferrous alloy iron and steel of afine even grain which is strong, durable, heat-resistant andcorrosion-resistant; one that possesses high tensile strength, and highimpact value; that lends itself to hot and cold working, hardening,polishing, and the like; and that is highly resistant to decarburizationand grain growth under the manyconditions of fabrication and use.

The invention; accordingly consists in the combination of elements,composition of ingredients, and mixture of materials, and in the severalsteps and therelation of each of the same to one or more of the othersas described herein, and the scope of the application of which isindicated in the following claims.

As conducive to a clearer understanding of certain features of myinvention it may at this point be noted that in heretofore known and/orused corrosion-resisting alloy irons and steels (iron-chromium alloyscontaining as essential ingredients approximately 10% to 30% chromium,.06%' to 1% carbon and the balance substantially iron) many highlydesirable characteristics are achieved. These alloys are durable,strong, and tough; they may be worked either cold or hot from strip,sheet or bar stock to give products or articles of desired size andshape; they are resistant to the corrosive effects of atmosphericconditions as well as to many acids, .alkalies and salts, and, finally,are resistant to the efiectsof high temperatures, resistingdiscoloration and scaling while retaining their physicalcharacteristics, strength, toughness and durability.

cold working, may be heat-treated to give a fair' range of hardness,strength, and impact resistance, and is not particularlysusceptible todecarburization and grain growth. The alloy, however.

In heretofore known and/or used alloys of the class indicated no onealloy, however, is equally is not qualified: for resistance to severelycorrosive conditions, for extreme high temperature duty, and especiallyfor high temperatures in corrosive media; which characterizes theironchromium alloys of high chromium contents.

Similarly, an iron-chromium alloy having a chromium content near theupper commercial limit for rustless ferrous alloys (about 27 to 30%)although highly resistant to corrosion, and re-' duction, in aninexpensive and eflicient manner,

of a corrosion resistant alloy iron or steel of an inherently-uniformfine grained texture; one

that is less subject to decarburization and grain growth, brittlenessand fatigue; one that is of improved workability over a wider range ofhot and cold working conditions, and over a wider range of chromiumcontents of the class of alloys described above; and one that, for theferritic alloys, lends itself to increased maximum hardening byheat-treatment and hardening by heattreatment throughout a wider rangeof chromium content giving an article or product of greater strengthwhich is more durable and of a higher impact value than in heretoforeknown and/or used alloys of the class indicated.

Referring now more particularly to the practice of my invention, to ironof desired carbon content there is added chromium, with or withoutsupplementary amounts of molybdenum, tungsten, vanadium, copper and thelike, together with small amounts of nitrogen giving acorrosion-r'esisting alloy iron or steel of an inherently fine grainstructure which is especially resistant to decarburization and graingrowth.

Illustrati'vely, the proportions of ingredients 7 added are such as togive a ferrous alloy analyzing approximately 10% to 30% chromium .06% to30% carbon, and .0'7% to .20% nitrogen. The

particular quantity ofJthe alloying metals togjether with the preciseamount of carbon and nitrogen present are largely determinant of thephysical characteristics of the alloy as will appear more fullyhereinafter.

Thus, 'ill'ustratively, 195 pounds of rustless iron scrap analyzingapproximately 17% chromium and .12% carbon, together with '77 pounds ofordinary low carbon scrap iron and 28 pounds of low-carbon ferrochromeanalyzing approximately 70% chromium and .10% carbon is charged into thecrucible of a 300-pound high-frequency induction furnace. A suitablerefractory lid, preferably having associated therewith a conduit bymeans of which gases may be either introduced or removed from thefurnace melting chamber, is then placed on the crucible. Next, thefurnace is started by energizing the furnace windings from a suitablesource of electrical energy.

The charge of scrap melts down giving a ferrous metal bath containingchromium and a small percentage of carbon. When this melt has beenbrought to a suitable temperature and a desired condition of refinement,nascent or atomic nitrogen (provided in any convenient manner as bypassing a stream of bottled nitrogen through one or more electric arcs)is introduced into the melting chamber, andin intimate contact with thebath of metal, thus displacing the normal furnace atmosphere in contactwith the surface of the bath of metal and providing a nascent nitrogenatmosphere for the further treatment of the metal bath.

Preferably, the nascent nitrogen is introduced by way of the conduit inthe lid of the furnace crucible and is permitted to flow through themelting chamber at atmospheric pressure, although under certainconditions the nitrogen atmosphere may be maintained either above orbelow this pressure. The nascent nitrogen is supplied for a period offrom fifteen minutes to one hour or more, depending upon the amount ofnitrogen desired in the final product, the percentage of chromiumcontent of the bath (which affects its afiinity for nitrogen), the bathtemperature, pressure of the gas, the chemical activity of the gas andthe like.

After final adjustment of bath temperature and alloy content (as byadding a small amount of low-carbon ferrochrome with or withoutsupplementary additions of molybdenum, tungsten, vanadium and. copper,together with small amounts of manganese and silicon) the heat is pouredinto suitable molds.

The tapped metal analyzes about 17.3% chromium, about .10% carbon, about.09% nitrogen, with the usual small amounts of manganese and silicon.

The properties conveyed by the nitrogen content are in some respectssimilar to the well known properties resulting from the use of a smallincreased amount of carbon without, however, the attendant undesirableeffects of carbon on corrosion resistance.

As indicated above, the alloy is of an inherently fine, even grainstructure and especially resistant to decarburization, this latterfeature becoming increasingly important in alloys of the higher chromiumcontents and especially those ofa chromium content between 16% and 20%.

Flowing from these inherent structural characteristics are manypractical advantages. Objectionable grain growth and decarburization ofthe rustless irons and steels of the higher chromium contents areappreciably decreased. The metal is more ductile, more" workable'andlends itself to hardening by heat-treatment over a wider range ofchromium content than in heretofore known rustless irons and steels.Cold forming operations, especially deep-drawing, are considerablyimproved while hot forming operations may be successfully carried outover a wider range of temperatures. Likewise, the tensile strength ofthe alloy may be appreciably increased throughout a broader chromiumrange. So, also, may the'impact resistance of the lower chromium alloysbe increased.

Furthermore, as a result of the fine grain structure and increasedresistance to grain growth mentioned above, the alloy more readilylends-it self to welding and, in addition, gives a weld of finer grainand hence one that is more reliable than in heretofore known alloys oftheclass indicated;

The hot working characteristics of the metal are not adversely affectedby the presence of nitrogen, the alloy lending itself to forging,upsetting, swaging and like hot operations, while the cold workingcharacteristics such as beading, spinning and-deep-drawing are somewhatimproved for all chromium analyses; the improvement in deep-drawingcharacteristics being particularly improved for a chromium content of Infact, as a result of the inherent resistance to decarburizationadvantage may be taken of higher working temperatures in processingingots to billets, sheet, strip and bars without resultantdecarburization, surface grain growth, and generally coarse structure.(The absence of decarburization and surface grain growth aids materiallyin producing a smooth bright surface which lends itself to easierpolishing, thereby effecting important economies in this costlyoperation.)

In addition to the many highly beneficial characteristics outlinedabove, the heat-resisting characteristics of the metal are considerablyimproved over heretofore known and/or used alloys of the class indicatedsince resistance to objectionable grain growth is inherently bettered asmore particularly pointed out above.

Other characteristics of alloys of the class indicated, such asresistance to corrosive effects of acid, alkaline and salt solutions arefully retained and, as a result of the finer grain structure, aresubstantially improved ,over heretofore known alloys of the kinddescribed.

While in the above illustrative embodiment of my invention theproduction of rustless irons and steels containing. 10% to 30% chromiumand .03% to .20% nitrogen is described, it will be understood thatiron-chromium alloys, or other alloy irons and steels, of the same orhigher nitrogen contents may be likewise produced without departing fromthe teachings set forth above.

Likewise, while in the embodiment illustratively set forth abovenitrogen containing rustless irons and steels are produced bymaintaining the molten metal in an induction furnace in the presence ofnascent nitrogen, it will be under-' stood that nitrogen containingrustless irons and steels or other iron-chromium alloys may be similarlyproduced by subjecting molten chromium containing iron or steel preparedin any well known manner, employing known furnacing methods, to anintimate association with nascent nitrogen; the nitrogen being passedonto or into the molten metal.

Thus it will be seen that there has been provided in this invention analloy rustless iron or steel, together with an art of producing thesame, in which the various objects hereinbefore noted together with manythoroughly practical advantages are successfully achieved. It will beseen that the physical characteristics of rustless ferrous alloys aregreatly improved; that these improved characteristics permit manysavings in manufacture and use heretofore unrealized; and that theuseful field of application of these alloys is appreciably broadened.

As many possible embodiments may be made of my invention and as manychanges may be made in the embodiment hereinbefore set forth. it is tobe understood that all matter described herein is to be interpreted asillustrative and not in a limiting sense.

I claim:

1. In the production of iron-chromium alloys of appreciable nitrogencontents, the art which includes preparing and maintaining a bath offerrous metal containing chromium in intimate contact with nascent oratomic nitrogen gas introduced into the presence of said bath.

2. In the production of iron-chromium alloys of appreciable nitrogencontents, the art which includes preparing abath of metal containingchromium, and passing nascent nitrogen'gas into intimate contact withsaid bath.

3. In the production of iron-chromium alloys of appreciable nitrogencontents, the art which includes preparing a chromium containing bath ofmetal, and passing nascent nitrogen gas over the surface of said bath.

4. In the production of rustless irons and steels of high nitrogencontents the art which includes, maintaining a bath of ferrous metalcontaining chromium in intimate contact with an atmosphere includingnascent nitrogen gas introduced into the presence of said bath.

5. In the production of'rustless irons and steels of high nitrogencontents, the art which includes, preparing a bath of ferrous metalcontaining chromium and introducing and maintaining nascent nitrogen gasin intimate contact with said bath until a desired amount of nitrogen istaken up by the bath to achieve a rustless iron or steel of desirednitrogen content.

6. In the production of iron-chromium alloys of high nitrogen contents,the art which includes, preparing a bath of ferrous metal containingchromium, passing nitrogen gas through one or more electric arcs formingthereby nascent or atomic nitrogen, and introducing said nascentnitrogen into intimate contact with said bath of metal.

7. In the production of rustless irons and steels of high nitrogencontents, the art which includes, preparing a bath of ferrous metalcontaining chromium, preparing nascent nitrogen by passing nitrogen gasthrough an electric arc, and introducing said nascent nitrogen into thepresence of said bath of metal whereby a desired nitrogen content isquickly achieved.

. WILLIAM B. ARNESS.

